Currently thermoplastic powder or thermoplastic slurries, known as dry or liquid plastisol, are cast on the inner surface of a thin metal mold member heated to the fusion temperature of the deposited material to form a gell coat which is cured to form a thin, solid thermoplastic shell. The thin metal mold member remains heated until a thin layer of thermoplastic material is built-up on the inner surface to form a thin plastic shell product suitable for use as an outer skin or shell covering on automotive interior products such as armrests, door panels, instrument panels and on automotive exterior products such as side rail bumpers, moldings or bumper fascias.
Examples of such heated mold assemblies and processes are shown in U.S. Pat. Nos. 3,680,629 and 4,389,177.
One form of heating is to direct hot gas over a flat outer surface of the heated mold to transfer heat into the metal of the mold for transfer therethrough into the thermoplastic material (sometimes referenced as plastisol material) to cause the plastisol (either wet or dry) to gel and cure. In the gel stage the plastisol is fused together into a thin layer of material. Thereafter the fused material is cured at an elevated temperature to form a finished product with desired physical characteristics. Examples of such gas heated molds are shown in U.S. Pat. Nos. 4,621,995 and 4,623,503.
In the past it has been recognized that the heat transfer rate for directing heat from the heat source to the deposited plastisol could be improved by providing fins on the outer surface of the mold and flowing heated air across the fins to increase heat transfer to the plastisol. The fins provided increased heat transfer area for flow of heat from the heat source to the plastisol. The attachment of the fins, however, required soldering, brazing or welding steps that produce excessive heat flow into the thin metal shell of the mold. Such past practices can cause distortion or thermal cracks in the mold during fabrication. Additionally such attachment methods can cause degradation of the properties of the metal in the vicinity of the connections.
An example of such property degradation is grain growth in the mold metal which makes the metal susceptible to cracking.